intensity and origin of the extragalactic gamma-ray ... · intensity and origin of the...
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Intensity and origin of the extragalactic gamma-ray background between 100 MeV and 820 GeV
Markus Ackermannon behalf of the Fermi LAT collaboration
5th Fermi Symposium, Nagoya20.10.2014 - 24.10.2014
Fermi LAT, 4-year sky map, E > 1 GeV
Markus Ackermann | 5th Fermi Symposium, Nagoya | 23/10/2014 | Page
The Fermi LAT gamma-ray sky
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Fermi LAT, 4-year sky map, E > 1 GeV
Markus Ackermann | 5th Fermi Symposium, Nagoya | 23/10/2014 | Page
The Fermi LAT gamma-ray sky
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Resolved sources
Fermi LAT, 4-year sky map, E > 1 GeV
Markus Ackermann | 5th Fermi Symposium, Nagoya | 23/10/2014 | Page
The Fermi LAT gamma-ray sky
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Resolved sources
Inverse Compton π0-decay
Bremsstrahlung
Galactic diffuse emission (CR interactions with the interstellar medium)
Fermi LAT, 4-year sky map, E > 1 GeV
Markus Ackermann | 5th Fermi Symposium, Nagoya | 23/10/2014 | Page
The Fermi LAT gamma-ray sky
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Resolved sources
Inverse Compton π0-decay
Bremsstrahlung
Galactic diffuse emission (CR interactions with the interstellar medium)
Isotropic diffuse emission (IGRB)
Markus Ackermann | 5th Fermi Symposium, Nagoya | 23/10/2014 | Page
The origin of the IGRB in the LAT energy range
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Undetected sources Diffuse processes
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Why is this important ?! The Extragalactic Gamma-ray Background may encrypt the signature of the
most powerful processes in astrophysics
Blazars contribute20-100% of theEGB (Stecker&Salomon96,Mücke&Pohl00,Narumoto&Totani04,Dermer07, Inoue&Totani09)
Emission from starforming galaxies (e.g.Pavlidou&Fields02)
Emission fromparticle acceleratedin Intergalacticshocks (Loeb&Waxmann00)
73% Dark Energy
23%Dar
k
Matter
4% A
tom
s
Emission due tothe annihilation ofCosmological DarkMatter (eg. Jungman+96)
Markevitch+05
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Why is this important ?! The Extragalactic Gamma-ray Background may encrypt the signature of the
most powerful processes in astrophysics
Blazars contribute20-100% of theEGB (Stecker&Salomon96,Mücke&Pohl00,Narumoto&Totani04,Dermer07, Inoue&Totani09)
Emission from starforming galaxies (e.g.Pavlidou&Fields02)
Emission fromparticle acceleratedin Intergalacticshocks (Loeb&Waxmann00)
73% Dark Energy
23%Dar
k
Matter
4% A
tom
s
Emission due tothe annihilation ofCosmological DarkMatter (eg. Jungman+96)
Markevitch+05
4
Why is this important ?! The Extragalactic Gamma-ray Background may encrypt the signature of the
most powerful processes in astrophysics
Blazars contribute20-100% of theEGB (Stecker&Salomon96,Mücke&Pohl00,Narumoto&Totani04,Dermer07, Inoue&Totani09)
Emission from starforming galaxies (e.g.Pavlidou&Fields02)
Emission fromparticle acceleratedin Intergalacticshocks (Loeb&Waxmann00)
73% Dark Energy
23%Dar
k
Matter
4% A
tom
s
Emission due tothe annihilation ofCosmological DarkMatter (eg. Jungman+96)
Markevitch+05
Blazars▪ Dominant class of LAT extra-
galactic sources. ▪ Estimated EGB contributions
ranging from 20% - 100%.
Radio galaxies▪ 27 sources listed in 2FGL. ▪ 25% - 50 % contribution to EGB
expected (large uncertainties!).
Star-forming galaxies▪ Several galaxies outside the
local group resolved by LAT. ▪ Significant contribution to EGB
expected.
GRBs + High-latitude pulsars▪ Small contributions expected.
Intergalactic shocks▪ Widely varying predictions of
EGB contribution ranging from 1% to 100%.
Dark matter annihilation▪ Potential signal dependent on
nature of DM, cross-section and structure of DM distribution.
Interactions of UHE cosmic rays with the EBL▪ Strongly dependent on evolution
of UHECR sources.▪ 1% - 100% of EGB emission.
Isotropic Galactic contributions ▪ Contributions from an extremely
large Galactic electron halo.▪ CR interaction in small solar
system bodies.
Markus Ackermann | 5th Fermi Symposium, Nagoya | 23/10/2014 | Page
The isotropic and the total extragalactic background
Intensity that can be resolved into sources depends on:
▪ the sensitivity of the instrument.▪ the exposure of the observation.
➞ The isotropic γ-ray background depends on the sensitivity to identify sources.
➞ Important as an upper limit on diffuse processes.
➞ The total extragalactic γ-ray background is instrument and observation independent.
➞ Useful for comparisons with source population models.
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+
Resolved sources
Isotropic γ-ray background (IGRB)
Total extragalactic γ-ray background (EGB)
Markus Ackermann | 5th Fermi Symposium, Nagoya | 23/10/2014 | Page
Derivation of the isotropic gamma-ray background
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=
+Solar disk and IC
+Resolved sources (2FGL)
+Isotropic emission
Interstellar gas
Inverse Compton (IC)
Loop I / Local Loop
Galactic diffuse emission
Not used in this analysis: > Galactic plane > Regions with dense molecular clouds > Regions with non-local
atomic hydrogen clouds
Low-energy analysis (100 MeV - 13 GeV) > High-statistics regime. > Intensity of all components is
fitted in each energy band.
High-energy analysis (13 GeV - 820 GeV) > Low-statistics regime > Only resolved sources and
isotropic emission are fitted in each energy band.
Markus Ackermann | 5th Fermi Symposium, Nagoya | 23/10/2014 | Page
Derivation of the isotropic gamma-ray background
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=
+Solar disk and IC
+Resolved sources (2FGL)
+Isotropic emission
Isotropic γ-ray back- ground (IGRB)
Contami- nation from CR induced background
Interstellar gas
Inverse Compton (IC)
Loop I / Local Loop
Galactic diffuse emission
Not used in this analysis: > Galactic plane > Regions with dense molecular clouds > Regions with non-local
atomic hydrogen clouds
Markus Ackermann | 5th Fermi Symposium, Nagoya | 23/10/2014 | Page
New event classifications for the EGB analysis
> Publicly available LAT event classes (P7ULTRACLEAN) have insufficient background rejection for this study at very low and very high energies.
> New high-purity event classes developed for this analysis.
> P7REP_IGRB_LO▪ Optimized to reject secondary
CR background at low energies> P7REP_IGRB_HI
▪ Optimized to reject primary CR background while retaining high statistics
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Low-energy analysis High-energy analysis
Markus Ackermann | 5th Fermi Symposium, Nagoya | 23/10/2014 | Page
Results from the IGRB fit
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Low-energy analysis High-energy analysis
Low-energy analysis High-energy analysis
> Based on 50 months of reprocessed LAT data.
> Average intensities ( |b| > 20° )attributed to model templates.
> Baseline foreground model used.
> IGRB and CR contributions to isotropic emission
> Spectral fit of IGRB by power-law with exponential cutoff.
Markus Ackermann | 5th Fermi Symposium, Nagoya | 23/10/2014 | Page
The IGRB spectrum
> Error bars:statistical error + syst. error from effective
area parametrization+ syst. error from CR
background subtraction
> Yellow band: systematic uncertainties from foreground model variations.
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> IGRB spectrum can be parametrized by single power-law + exponential cutoff.> Spectral index ~ 2.3 , cutoff energy ~ 250 GeV.> It is not compatible with a simple power-law (χ2 > 85).
Low-energy analysis High-energy analysis
different models for Galactic foreground.
Markus Ackermann | 5th Fermi Symposium, Nagoya | 23/10/2014 | Page
The total extragalactic background
> Sum of the intensities of IGRB and the resolved high-latitude sources.> Contribution of high-latitude Galactic sources << 5%.> Spectrum can be parametrized by power-law with exponential cutoff.> Spectral index ~ 2.3, cutoff energy ~ 350 GeV.
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Markus Ackermann | 5th Fermi Symposium, Nagoya | 23/10/2014 | Page
Comparison of LAT IGRB and EGB measurements
> Comparison for baseline diffuse model.> Integrated intensity of IGRB about 30% below measurement in Abdo et al. 2010.> Compatible within systematic uncertainties.> Main differences: Improved diffuse foreground and CR background models.
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Markus Ackermann | 5th Fermi Symposium, Nagoya | 23/10/2014 | Page
Comparison to other experiments
> Cosmic x-ray and gamma-ray background now measured over 9 orders of magnitude in energy.
> Is the observed cutoff compatible with an absorption feature ?
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Markus Ackermann | 5th Fermi Symposium, Nagoya | 23/10/2014 | Page
The shape of the high-energy IGRB spectrum
> Simple population of sources with power-law spectrum with index Γ
> Luminosity or density evolution ~ (1+z)β or following star-formation rate
> Observed EGB spectrum is compatible with single population of sources with power-law spectrum (Γ=2.3) and no evolution (β=0).
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Franceschini et al., 2008 EBL model
Markus Ackermann | 5th Fermi Symposium, Nagoya | 23/10/2014 | Page
Source populations contributing to the EGB
> … but reality might be more complex.> Multiple populations might contribute to explain the observed spectrum.> Blazars seem to dominate above few GeV.
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PRELIMINARY
submitted to ApJL
Markus Ackermann | 5th Fermi Symposium, Nagoya | 23/10/2014 | Page
Summary
> The spectrum of the isotropic and total extragalactic gamma-ray background was measured between 100 MeV and 820 GeV.▪ Energy range of measurement in Abdo et al. 2010 extended by more than an order of
magnitude.▪ Paper accepted by ApJ (The Fermi LAT Collaboration, arXiv:1410.3696).
> The IGRB spectrum can be described over the full energy range by a simple power law of index ~2.3 with an exponential cutoff at ~250 GeV.
> First clear evidence for cutoff at high energies.
> The shape of the cutoff is compatible with expectations due to absorption of the gamma rays in the extragalactic background light.
> Uncertainty in diffuse foreground modeling is the largest systematic uncertainty for the IGRB measurement. Future work needs to address this.
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